Merge clangd's inactiveRegions with existing diagnostics (#26737)

Closes: https://github.com/zed-industries/zed/issues/13089

This PR attempts to resolve the issues discussed in my previous PR
#26146.

Release Notes:

- Fixed: `inactiveRegions` doesn't replace existing diagnostics anymore

crates/language/src/language.rs

@@ -229,8 +229,14 @@ impl CachedLspAdapter {
         self.adapter.code_action_kinds()
     }
 
-    pub fn process_diagnostics(&self, params: &mut lsp::PublishDiagnosticsParams) {
-        self.adapter.process_diagnostics(params)
+    pub fn process_diagnostics(
+        &self,
+        params: &mut lsp::PublishDiagnosticsParams,
+        server_id: LanguageServerId,
+        existing_diagnostics: Option<&'_ Buffer>,
+    ) {
+        self.adapter
+            .process_diagnostics(params, server_id, existing_diagnostics)
     }
 
     pub async fn process_completions(&self, completion_items: &mut [lsp::CompletionItem]) {
@@ -443,7 +449,13 @@ pub trait LspAdapter: 'static + Send + Sync {
         delegate: &dyn LspAdapterDelegate,
     ) -> Option<LanguageServerBinary>;
 
-    fn process_diagnostics(&self, _: &mut lsp::PublishDiagnosticsParams) {}
+    fn process_diagnostics(
+        &self,
+        _: &mut lsp::PublishDiagnosticsParams,
+        _: LanguageServerId,
+        _: Option<&'_ Buffer>,
+    ) {
+    }
 
     /// Post-processes completions provided by the language server.
     async fn process_completions(&self, _: &mut [lsp::CompletionItem]) {}
@@ -2060,8 +2072,6 @@ impl LspAdapter for FakeLspAdapter {
         unreachable!();
     }
 
-    fn process_diagnostics(&self, _: &mut lsp::PublishDiagnosticsParams) {}
-
     fn disk_based_diagnostic_sources(&self) -> Vec<String> {
         self.disk_based_diagnostics_sources.clone()
     }